Bacterial protein can replace half of the fishmeal in gilthead seabream diets without affecting growth

The search for alternative protein sources for aquafeeds continues to focus on ingredients that can reduce dependence on fishmeal without compromising fish performance. Among the options attracting increasing interest is single-cell protein derived from methanotrophic bacteria, microorganisms capable of using methane as a carbon source to produce protein-rich biomass.

However, one of the main questions facing feed manufacturers is how far these ingredients can replace conventional raw materials without affecting growth, feed efficiency or fish health.

In gilthead seabream (Sparus aurata), a 96-day experimental study published in Aquaculture, led by Enric Gisbert and an international research team, demonstrated that replacing 50% of the fishmeal by incorporating 85 grams of bacterial protein per kilogram of feed maintained growth and feed efficiency compared with a reference diet containing no bacterial protein. Higher replacement levels of 75% and 100%, however, reduced final fish weight by 9.1% and 20.1%, respectively.

The researchers suggest that increasing replacement beyond this level reduces performance, potentially because of a nutritional imbalance associated with lower levels of DHA and other omega-3 fatty acids in the diets containing higher proportions of microbial protein, reduced feed acceptance by the fish, or a combination of both factors.

Specifically, increasing fishmeal replacement to 75% and 100% reduced final fish weight by 9.1% and 20.1%, respectively. The researchers also examined the intestinal microbiota and the response of fish to Vibrio harveyi, finding that survival decreased at higher inclusion levels.

These observations reinforce the need to evaluate new ingredients as part of a complete diet, rather than solely on the basis of their protein content or digestibility.

For feed manufacturers and gilthead seabream producers, the next question is whether replacing fishmeal with bacterial protein can be economically competitive under commercial farming conditions.

Beyond ingredient cost and industrial availability, it will also be necessary to establish whether the results can be maintained over longer production cycles and whether the limitations observed at higher inclusion levels can be addressed through adjustments to feed formulation.

For now, the research provides further evidence of the potential of single-cell proteins, while also highlighting that the success of an alternative ingredient should not be measured by how much conventional raw material it can replace, but by the performance it allows the finished feed to maintain.

Scientific reference: Gisbert, E. et al. (2026). Bacterial single cell protein from methanotrophic bacteria as an alternative ingredient for gilthead seabream (Sparus aurata) aquafeeds: Effects on digestibility, growth performance, gut microbiota, and disease resistance. Aquaculture, 625, 744316. DOI: 10.1016/j.aquaculture.2026.744316.